The present invention relates to expandable endoprosthesis devices, generally known as stents, which are designed for implantation in a patient""s body lumen, such as blood vessels to maintain the patency thereof. These devices are particularly useful in the treatment and repair of blood vessels after a stenosis has been compressed by percutaneous transluminal coronary angioplasty (PTCA), percutaneous transluminal angioplasty (PTA), or removed by atherectomy or other means.
Stents are generally cylindrically-shaped devices which function to hold open and sometimes expand a segment of a blood vessel or other lumen such as a coronary artery. They are particularly suitable for use to support the lumen or hold back a dissected arterial lining which can occlude the fluid passageway therethrough.
A variety of devices are known in the art for use as stents and have included coiled wires in a variety of patterns that are expanded after being placed intraluminally on a balloon catheter; helically wound coiled springs manufactured from an expandable heat sensitive metal; and self-expanding stents inserted in a compressed state and shaped in a zigzag pattern. One of the difficulties encountered using prior art stents involved maintaining the radial rigidity needed to hold open a body lumen while at the same time maintaining the longitudinal flexibility of the stent to facilitate its delivery and accommodate the often tortuous path of the body lumen.
Another problem area has been the limited range of expandability. Certain prior art stents expand only to a limited degree due to the uneven stresses created upon the stents during radial expansion. This necessitates providing stents with a variety of diameters, thus increasing the cost of manufacture. Additionally, having a stent with a wider range of expandability allows the physician to redilate the stent if the original vessel size was miscalculated.
Another problem with the prior art stents has been contraction of the stent along its longitudinal axis upon radial expansion of the stent. This can cause placement problems within the artery during expansion.
Various means have been described to deliver and implant stents. One method frequently described for delivering a stent to a desired intraluminal location includes mounting the expandable stent on an expandable member, such as a balloon, provided on the distal end of an intravascular catheter, advancing the catheter to the desired location within the patient""s body lumen, inflating the balloon on the catheter to expand the stent into a permanent expanded condition and then deflating the balloon and removing the catheter.
What has been needed is a stent which not only addresses the aforementioned problems, but also has variable strength, yet maintains flexibility so that it can be readily advanced through tortuous passageways and radially expanded over a wider range of diameters with minimal longitudinal contraction to accommodate a greater range of vessel diameters, all with minimal longitudinal contraction. Certainly, the expanded stent must have adequate structural strength (hoop strength) to hold open the body lumen in which it is expanded. The control of stent strength at specific locations along the stent results in a highly customizable device specifically adapted to the unique body lumen formation in the patient.
One approach to the variable strength problem is to increase strut thickness. This technique is disclosed in co-pending application Ser. No. 08/943,992, filed Oct. 3, 1997, by T. Limon and T. Tumlund, entitled xe2x80x9cStent Having Varied Amounts Of Structural Strength Along Its Length,xe2x80x9d whose entire contents are hereby incorporated by reference. Another approach is to vary the length or width of the strut at a constant strut thickness. The present invention is directed to this approach.
The present invention is directed to stents of enhanced longitudinal flexibility and configuration which permit the stents to expand radially to accommodate a greater number of different diameter vessels, both large and small, than heretofore was possible. The stents of the instant application also have greater flexibility along their longitudinal axis to facilitate delivery through tortuous body lumens, but remain highly stable when expanded radially, to maintain the patency of a body lumen such as an artery or other vessel when implanted therein. The unique patterns of the stents of the instant invention permit both greater longitudinal flexibility and enhanced radial expansibility and stability compared to prior art stents.
Each of the different embodiments of stents of the present invention includes a plurality of adjacent cylindrical elements which are generally expandable in the radial direction and arranged in alignment along a longitudinal stent axis. The cylindrical elements are formed in a variety of serpentine wave patterns transverse to the longitudinal axis and contain a plurality of alternating peaks and valleys. At least one interconnecting member extends between adjacent cylindrical elements and connects them to one another. These interconnecting members insure minimal longitudinal contraction during radial expansion of the stent in the body vessel. The serpentine patterns have varying degrees of curvature in the regions of the peaks and valleys and are adapted so that radial expansion of the cylindrical elements are generally uniform around their circumferences during expansion of the stents from their contracted conditions to their expanded conditions.
The resulting stent structures are a series of radially expandable cylindrical elements that are spaced longitudinally close enough so that small dissections in the wall of a body lumen may be pressed back into position against the lumenal wall, but not so close as to compromise the longitudinal flexibility of the stent both when being negotiated through the body lumens in their unexpanded state and when expanded into position. The serpentine patterns allow for an even expansion around the circumference by accounting for the relative differences in stress created by the radial expansion of the cylindrical elements. Each of the individual cylindrical elements may rotate slightly relative to their adjacent cylindrical elements without significant deformation, cumulatively providing stents which are flexible along their length and about their longitudinal axis, but which are still very stable in the radial direction in order to resist collapse after expansion.
Each of the stents of the present invention can be readily delivered to the desired lumenal location by mounting it on an expandable member, such as a balloon, of a delivery catheter and passing the catheter-stent assembly through the body lumen to the implantation site. A variety of means for securing the stents to the expandable member of the catheter for delivery to the desired location are available. It is presently preferred to compress or crimp the stent onto the unexpanded balloon. Other means to secure the stent to the balloon include providing ridges or collars on the inflatable member to restrain lateral movement, using bioabsorbable temporary adhesives, or adding a retractable sheath to cover the stent during delivery through a body lumen.
The presently preferred structures for the expandable cylindrical elements which form the stents of the present invention generally have a circumferential serpentine pattern containing a plurality of alternating peaks and valleys. The degrees of curvature along adjacent peaks and valleys are designed to compensate for the stresses created during expansion of the stent so that expansion of each of the peaks and valleys is uniform relative to one another. This particular structure permits the stents to radially expand from smaller first diameters to any number of larger second diameters since stress is distributed more uniformly along the cylindrical elements. This uniformity in stress distribution reduces the tendency of stress fractures in one particular region and allows high expansion ratios.
The different stent embodiments also allow the stents to expand to various diameters from small to large to accommodate different-sized body lumens, without loss of radial strength and limited contraction of longitudinal length. The open reticulated structure of the stents results in a low mass device. It also enables the perfusion of blood over a large portion of the arterial wall, which can improve the healing and repair of a damaged arterial lining.
The serpentine patterns of the cylindrical elements can have different degrees of curvature of adjacent peaks and valleys to compensate for the expansive properties of the peaks and valleys. Additionally, the degree of curvature along the peaks can be set to be different in immediately adjacent areas to compensate for the expansive properties of the valleys adjacent to it. The more even radial expansion of this design results in stents which can be expanded to accommodate larger diameters with minimal out of plane twisting since the high stresses are not concentrated in any one particular region of the pattern, but are more evenly distributed among the peaks and valleys, allowing them to expand uniformly. Reducing the amount of out of plane twisting also minimizes the potential for thrombus formation.
The serpentine pattern of the individual cylindrical elements can optionally be in phase which each other in order to reduce contraction of the stents along their length when expanded. The cylindrical elements of the stents are plastically deformed when expanded (except with NiTi alloys) so that the stents will remain in the expanded condition and therefore they must be sufficiently rigid when expanded to prevent the collapse thereof in use.
With stents formed from super-elastic nickel-titanium (NiTi) alloys, the expansion occurs when the stress of compression is removed. This allows the phase transformation from martensite back to austenite to occur, and as a result the stent expands.
After the stents are expanded, some of the peaks and/or valleys may, but not necessarily, tip outwardly and embed in the vessel wall. Thus, after expansion, the stents might not have a smooth outer wall surface. Rather, they might have small projections which embed in the vessel wall and aid in retaining the stents in place in the vessel. The tips projecting outwardly and strut twisting are due primarily to the struts having a high aspect ratio. In one preferred embodiment, the strut width is about 0.0035 inch and a thickness of about 0.0022 inch, providing an aspect ratio of 1.6. An aspect ratio of 1.0 will produce less tipping and twisting.
The elongated interconnecting members which interconnect adjacent cylindrical elements should have a transverse cross-section similar to the transverse dimensions of the undulating components of the expandable cylindrical elements. The interconnecting members may be formed in a unitary structure with the expandable cylindrical elements formed from the same intermediate product, such as a tubular element, or they may be formed independently and mechanically secured between the expandable cylindrical elements.
Preferably, the number and location of the interconnecting members can be varied in order to develop the desired longitudinal flexibility in the stent structure both in the unexpanded as well as the expanded condition. These properties are important to minimize alteration of the natural physiology of the body lumen into which the stent is implanted and to maintain the compliance of the body lumen which is internally supported by the stent. Generally, the greater the longitudinal flexibility of the stents, the easier and the more safely they can be delivered to the implantation site, especially where the implantation site is on a curved section of a body lumen, such as a coronary artery or a peripheral blood vessel, and especially saphenous veins and larger vessels.
Following from the foregoing proposition is that, in general, the more interconnecting members there are between adjacent cylindrical elements of the stent, the less longitudinal flexibility there is. More interconnecting members reduces flexibility, but also increases the coverage of the vessel wall, which helps prevent tissue prolapse between the stent struts. Such an approach to stent design is disclosed in co-pending patent application Ser No. 09/008,366, filed Jan. 16, 1999, by Daniel L. Cox, entitled xe2x80x9cFlexible Stent And Method of Use,xe2x80x9d whose entire contents are hereby incorporated by reference.
The present invention in particular relates to the control of stent strength by varying the strut geometry along the length of the stent. By making the stent stronger or weaker in different regions of the stent, the properties can be customized to a particular application. The stent properties that could be altered include, but are not limited to, the width of each strut, and/or the length of each cylindrical element or ring at a constant strut thickness.
The variation of the strength of the stent affects the manner in which the stent expands. As expected, the wider struts tend not to deform as easily as the narrower struts during expansion, while the longer struts within the longer cylindrical elements are better adapted to deployment in larger diameter vessels. On the other hand, an area with shorter cylindrical elements tends to have greater radial strength than an area with longer cylindrical elements, given the same strut cross-sectional area.
In a preferred embodiment, the present invention is directed to a longitudinally flexible stent for implanting in a body lumen and which is expandable from a contracted condition to an expanded condition. The present invention stent preferably comprises a plurality of adjacent cylindrical elements, each cylindrical element having a circumference extending around a longitudinal stent axis, being substantially independently expandable in the radial direction, wherein the plurality of adjacent cylindrical elements are arranged in alignment along the longitudinal stent axis and define a first end section, a second end section, and a center section therebetween; each cylindrical element having struts of a constant thickness formed in a generally serpentine wave pattern transverse to the longitudinal axis and containing alternating valley portions and peak portions; a plurality of interconnecting members extending between the adjacent cylindrical elements and connecting valley portions of adjacent cylindrical elements to one another; and wherein the struts of at least one cylindrical element has greater mass than the struts in other cylindrical elements.
The greater mass strut is achieved by increasing the length of the strut, and/or increasing the width of the strut. On the other hand, the greater mass strut is not achieved by increasing strut thickness.
In an exemplary embodiment, the present invention stent has struts in the cylindrical elements in the center section that have a greater mass than the struts in the cylindrical elements in the first end section and the second end section. The greater mass is achieved by increasing strut width and/or increasing strut length.
In another exemplary embodiment, the present invention stent has struts in the cylindrical elements in the center section and the second end section that have a greater mass than the struts in the cylindrical elements in the first end section. The greater mass struts is achieved by increasing strut width and/or increasing strut length.
Still another exemplary embodiment of the present invention stent includes struts in the first end section and the second end section having greater mass than the struts in the cylindrical elements in the center section. The greater mass struts is achieved by increasing strut width and/or increasing strut length.
Increasing or decreasing strut length in each section changes the moment arm and consequently the radial strength of that section. Increasing or decreasing strut width at a constant strut thickness changes the cross-sectional area of the strut and the bending moment. Hence, a wider strut has greater hoop strength and is more resistant to bending.
Other features and advantages of the present invention will become more apparent from the following detailed description of the invention, when taken in conjunction with the accompanying exemplary drawings.